What is the equation for destructive interference?

The last term in the total irradiance equation from optical physics is known as the "interference term". That interference term will indicate the total magnitude of interference and the sign will indicate whether the interference is destructive (-) or constructive (+). The total irradiance equation is: Itot = I1 + I2 + 2*SQRT(I1*I2)cos(A) where angle 'A' is the phase angle between the E-fields of the two interfering waves. If 'A' is greater than 90 degrees and less than or equal to 180 degrees, the interference is destructive. Note that if 'A' equals 90 degrees, there is zero interference. If 'A' is between 0 degrees and 90 degrees, the interference is constructive.